Tare Weight
The empty cylinders of 20, 30 or 40 pounds capacity feature the same diameter. But the height varies a few inches. Therefore, the tare weights of these are 5-6 pounds distant from each other. Here is the list-
Tare Weight of 20 lbs propane tank: 16.6 pounds.
Tare Weight of 30 lbs propane tank: 23.5 pounds.
Tare Weight of 40 lbs propane tank: 29 pounds.
Propane Weight
Propane weight of 20 lbs propane tank: 19.8 pounds
Propane weight of 30 lbs propane tank: 30.8 pounds
Propane weight of 40 lbs propane tank: 41.36 pounds
Full Weight
Full weight of 20 lbs propane tank: 36.4 pounds
Full weight of 30 lbs propane tank: 54.5 pounds
Full weight of 40 lbs propane tank: 70.36 pounds
Link to above and more info:
https://camper101.com/20-lb-vs-30-lb...e-tank-for-rv/